1. Field of the Invention
The present invention relates to a permanent magnet alloy, as well as a magnet made thereof, that is based on a rare-earth element (R), iron (Fe), boron (B) and carbon (C) or that is based on a rare-earth element (R), iron (Fe), cobalt (Co), boron (B) and carbon (C) and that has improved resistance to oxidation. The invention also relates to a process for producing such an alloy or a magnet. The term "permanent magnet alloy" herein used means a magnetic alloy which is adapted for making a permanent magnet.
2. Background Information
Since its first disclosure by Sagawa et al., a magnet based on the R--Fe--B system has been the subject of many reports principally because it has the potential to be used as a next-generation magnet that surpasses Sm--Co based magnets in terms of magnetic force produced. However, though that magnet surpasses Sm--Co based magnets in terms of magnetic force, the heat stability of the magnetic characteristics and oxidation resistance of the new magnet are far inferior to those of said prior art magnets. For instance, the permanent magnet material described in Japanese Patent Public Disclosure No. 59-46008 is not capable of withstanding use in practical applications.
Many of the reports on said new magnets that have been published to data point out their shortcomings in regard of oxidation resistance and propose various methods for improvement, which are roughly divided into two categories, one based on modifying alloy compositions and the other based on covering the surface of magnets with an oxidation-resistant protective film. As an example of the methods of the first approach, Japanese Patent Public Disclosure No. 59-64733 teaches that a magnet can be made corrosion-resistant by replacing part of Fe with Co. Japanese Patent Disclosure No. 63-114939 teaches that improved oxidation resistance can be provided by incorporating in the matrix phase a low melting metal element such as Al, Zn or Sn or a high melting metal element such as Fe, Co or Ni. Further, Japanese Patent Public Disclosure Nos. 62-133040 and 63-77103 show that C (carbon) in a magnet promotes its oxidation and hence its oxidation resistance can be improved by reducing the C content to a level below a certain limit.
However, the effectiveness of these methods which solely depend upon the modification of alloy compositions for improving the resistance to oxidation is limited and it is difficult to produce magnets that reasonably withstand use in practical applications. Under these circumstances, it is necessary to manufacture a practicable magnet by coating its surface (the outermost exposed surface of the magnet) with an oxidation-resistant protective film through many complicated steps as shown in Japanese Patent Public Disclosure No. 63-114939.
It has been proposed that the oxidation-resistant protective film be formed on the surface of a magnet by covering it with an oxidation-resistant material by various methods such as plating, sputtering, evaporation and coating of organic materials. However, in each of these cases, a rugged and homogeneous protective film layer must be formed in a thickness of at least several tens of .mu.ms on the outer surface of the magnet. The procedure of forming such a thick layer requires many and complicated steps, which unavoidably results in such problems as spalling, low dimensional accuracy and increased production cost.
As described above, the existing R--Fe--B, R--Fe--Co--B and R--Fe--Co--B--C based magnets are not completely satisfactory in their ability to resist oxidation. As a matter of fact, these magnets have superior magnetic characteristics over Sm--Co based magnets and in addition, they have a great advantage in that they can be supplied consistently from abundant resources. However, these magnets cannot be put to practical use unless they are insulated from the operating atmosphere by means of an oxidation-resistant protective film formed on their surface and the above-described great advantage of these magnets is substantially compromised by the increased production cost and such problems as variations in dimensional accuracy.
A magnet based on R--Fe--B system is generally composed of magnetic crystal grains and a non-magnetic phase including a B-rich phase and a Nd-rich phase. A plausible explanation for the mechanism of oxidation that occurs in the magnet is that oxidation starts in the B-rich phase on either the magnet surface or in a nearby area and proceeds into the Nd-rich phase. Thus, it can be concluded that in order to improve the oxidation resistance of the magnet, it is necessary that not only the B content be reduced to the lowest possible level but also oxidation resistance be imparted to the Nd-rich phase. However, with the state of the art, the B content must inevitably be increased in order to attain magnetic characteristics of high practical levels, and no significant results have been achieved in the efforts to impart oxidation resistance to the Nd-rich phase.
As already mentioned, Japanese Patent Public Disclosure No. 59-64733 proposes that corrosion resistance be imparted by replacing part of Fe with Co but it makes no mention at all of the relevancy of the B content to oxidation resistance. The only disclosure given in this patent in regard of the B content is as follows: the B content is adjusted to lie within the range of 2-28 wt. % in order to secure a coercive force (iHc) of at least 1 kOe; in order to insure iHc of 3 kOe, the B content must be at least 4 at. %; and in order to attain high practical levels of iHc, the B content is further increased. However, if boron is to be contained in an increased amount with a view to attaining high magnetic characteristics, it is very difficult in practice to secure satisfactory oxidation resistance even if corrosion resistance is imparted by adding Co. Hence, in order to make a commercial magnet having high B content, it is essential to form a rugged oxidation-resistant protective film on the surface (the outermost exposed surface) of a magnet as taught by the inventors of the invention described in the Japanese Patent Public Disclosure mentioned at the beginning of this paragraph.
Japanese Patent Public Disclosure No. 63-114939 teaches the inclusion of a low melting metal element (e.g. Al, Zn or Sn) or a high melting metal (e.g. Fe, Co or Ni) in the matrix phase in order to improve the oxidation resistance of the active Nd-rich phase. According to an example shown in this patent, a weathering test (60.degree. C..times.90% RH) was conducted on a sinter and the period of time for which it could be left to stand until red rust developed noticeably on the surface of the magnet was prolonged to 100 h from 25 h which was the value for a comparative sample. However, the magnet having this level of oxidation resistance is not suitable for use in practical situations unless the surface of the magnet is protected by a rugged oxidation-resistant film. Thus, in this case, too, it is difficult to achieve a substantial improvement in the oxidation resistance of the magnet per se. It should also be noted that this Japanese Patent Public Disclosure makes no mention at all of the B content with regard to oxidation resistance and in the light of the B content which ranges from 3.5 to 6.7 at. % that is specified in the examples, one may safely conclude that the inclusion of B within the range of 2-28 at. % as set forth in Japanese Patent Public Disclosure No. 59-46008 is also contemplated by this publication.